1. Field of the Invention
This invention relates generally to a DAA interface (e.g., a Data Access Arrangement for a modem, telephone answering device, etc.) More particularly, it relates to a digital gyrator method and apparatus which emulates an inductor value of a telephone line interface on a telephone line in a stable manner after going into an off-hook condition.
2. Background of Related Art
Telephone systems in countries throughout the world have unique system requirements that need to be followed in order to legally sell and use telecommunication devices within their respective borders. One of the commonly known system requirements mandates that when a telephone line goes off-hook (i.e., when the telephone line is in use), the DC current level on the line must reach a certain level within a specified period of time and maintain that level until the call is completed. The DC current level, on the line must stay at a certain level in order to be interpreted by the telephone system as an active line throughout the duration of the telephone call. The current rise time and maximum current level are also regulated to prevent damage to telecommunication equipment.
In order to hold a telephone line in the off-hook condition, a specified level of current must be drawn which relates to the voltage level on the line and conforms to a country's telecommunication requirements. The desired operating current is generally expressed on a graph of current-versus-voltage known in the art as a load-line. The load-line represents a level of resistance for voltages on a current-versus-voltage graph, allowing a level of current to be determined for a given voltage.
FIG. 15 is an example of a current-versus-voltage load-line requirement to keep a telephone line in an off-hook condition. The slope of the load-line on a current-versus-voltage graph is the inverse of the line resistance. Telephone systems develop a voltage which is a potential impressed on the telephone line between two terminals, commonly known as the tip and ring voltage. As seen in FIG. 15, the desired level of current to keep a telephone line in the off-hook condition can be achieved for a given voltage by setting an appropriate line resistance. The template illustrated in FIG. 15 is representative of the parameters set forth by a country and varies from country to country. The parameters can even change within a country due to changes in a country's requirements (e.g., if a country updates their telecommunication system).
One method that has been used to set the DC line current on a telephone line when the telephone line goes off-hook is to place an inductor in series with a resistor across a telephone line connection and then couple the voice circuits to the line through a capacitor.
FIG. 16 shows a conventional circuit for setting DC line current.
In particular, as shown in FIG. 16, a commonly known prior art circuit for setting DC line current comprises resistance RDC, capacitance C and inductance L. Since inductors appear as shorts to DC current and as high impedance to AC current, the AC current is filtered out leaving just the DC current on the telephone line. The DC current can be set by choosing an appropriate value for the resistance RDC, dependent on the particular DC current level required. The circuit of FIG. 16 is less than optimal because of the inherently bulky nature and high cost of the inductor L, the amount of time for inductor L to charge, and the need to change circuit elements in countries with different off-hook current level requirements.
Voice band modems must present the same electrical characteristics to a telephone line as a standard telephone to meet international requirements allowing attachment of the voice band modem to the telephone line. One conventional approach that has been used to control the DC line current in a telephone system such as a voice band modem replaces the inductor L of FIG. 16 with additional system components that are smaller and less expensive to emulate the large inductor used in older telephone sets to set the loop DC resistance independent of the voice band impedance.
FIG. 17 shows a conventional gyrator circuit.
As shown in FIG. 17, such an arrangement of components can be used to control DC line current and is commonly known in the industry as a gyrator. A gyrator is a circuit used in a modem to set the DC termination while not disturbing the AC signals for transmit and receive. A gyrator allows a voice band modem to be reduced in cost and size, while still meeting the signal linearity needs of V.90 and V.34 voice band modems.
The conventional gyrator depicted in FIG. 17 functions like a large inductor across the telephone line and can be used in place of the conventional discrete component circuit shown in FIG. 16. The conventional gyrator is implemented with many discrete components such as transistors, resistors, capacitors, and digitally controlled switches located close to the tip and ring of the telephone line interface. As shown in FIG. 17, the conventional gyrator contains digitally controlled switches DCSC and DCSR used to switch different levels of capacitance and resistance into the gyrator circuit, respectively. By switching different levels of capacitance and resistance into the circuit, the time constant of the circuit can be changed, such that the transistors can be manipulated to provide the correct level of current on the telephone line within a specified period of time. The circuit allows different start up transient times and DC current levels to be adjusted in accordance with a user's specifications using a single circuit. The DCSC switches affect initial transient settling time and the DCSR switches affect the DC load-line. However, the adjustability of the circuit is set when the circuit is manufactured, limited by the physical components used in the circuit. If the specifications change after manufacture, in order to change the device, components need to be physically changed within the device or an entirely new device needs to be installed.
FIG. 18A depicts another V/I load line, and FIG. 18B shows a basic gyrator design which is implemented with external circuitry.
As shown in FIG. 18B, the gyrator appears as 400 ohms at DC and a large impedance for any voice band modem signal from 50 Hz to 4 kHz. The gyrator uses transistors and small capacitors to emulate the large inductor shown in FIG. 18B in a much smaller size and cost than the physical inductor. For the example shown in FIG. 18B, inductors are used to simplify the schematic. In the example, the DC voltage across the modem VTIP is set by the DC current flowing through resistor RP. The transfer function from VCO to VRP is as follows:                     V        ⁢                                   ⁢        R        ⁢                                   ⁢                  P          ⁡                      (            f            )                                      V        ⁢                                   ⁢        C        ⁢                                   ⁢                  O          ⁡                      (            f            )                                =                  H        ⁡                  (          f          )                    =                                    R            ⁢                                                   ⁢            P                                              R              ⁢                                                           ⁢              C              ⁢                                                           ⁢              O                        +                          s              ×              L              ⁢                                                           ⁢              S                        +                          R              ⁢                                                           ⁢              P                                      =                                            R              ⁢                                                           ⁢              P                                      (                                                R                  ⁢                                                                           ⁢                  P                                +                                  R                  ⁢                                                                           ⁢                  C                  ⁢                                                                           ⁢                  O                                            )                                ×                      1                          1              +                              s                ×                                                      L                    ⁢                                                                                   ⁢                    S                                                                              R                      ⁢                                                                                           ⁢                      P                                        +                                          R                      ⁢                                                                                           ⁢                      C                      ⁢                                                                                           ⁢                      O                                                                                                                          F      ⁢                           ⁢      c        =                                        R            ⁢                                                   ⁢            P                    +                      R            ⁢                                                   ⁢            C            ⁢                                                   ⁢            O                                    2          ×          π          ×          L          ⁢                                           ⁢          S                    =              3.71        ⁢                                   ⁢        Hz            
This accomplishes the task of separating the AC and DC terminations. The low corner frequency is needed to minimize degradation of the modem signal by the gyrator. This low corner frequency comes at the price of slow settling time. Solving the differential equation for VCO(t) in terms of VRP(t), the voltage across the 400 Ohm resistor, yields the time constant τ. To simplify the analysis, the loading of CAC is ignored. The time constant τ is equal to the coefficient of the first derivative.VCO(t)=RCO×ICO(t)+LS×ICO(t)+RP×ICO(t)VRP(t)=RP×ICO(t)            V      ⁢                           ⁢      L      ⁢                           ⁢              S        ⁡                  (          t          )                      =          L      ⁢                           ⁢      S      ×                                    ⅆ            I                    ⁢                                           ⁢          C          ⁢                                           ⁢          O                          ⅆ          t                                I      ⁢                           ⁢      C      ⁢                           ⁢              O        ⁡                  (          t          )                      =                  V        ⁢                                   ⁢        R        ⁢                                   ⁢                  P          ⁡                      (            t            )                                      R        ⁢                                   ⁢        P                        V      ⁢                           ⁢      C      ⁢                           ⁢              O        ⁡                  (          t          )                      =                            (                                    R              ⁢                                                           ⁢              P                        +                          R              ⁢                                                           ⁢              C              ⁢                                                           ⁢              O                                )                          R          ⁢                                           ⁢          P                    ×              (                                                            L                ⁢                                                                   ⁢                S                                            (                                                      R                    ⁢                                                                                   ⁢                    P                                    +                                      R                    ⁢                                                                                   ⁢                    C                    ⁢                                                                                   ⁢                    O                                                  )                                      ×                                                            ⅆ                  V                                ⁢                                                                   ⁢                R                ⁢                                                                   ⁢                P                                            ⅆ                t                                              +                      V            ⁢                                                   ⁢            R            ⁢                                                   ⁢                          P              ⁡                              (                t                )                                                    )                  τ    =                            L          ⁢                                           ⁢          S                          (                                    R              ⁢                                                           ⁢              P                        +                          R              ⁢                                                           ⁢              C              ⁢                                                           ⁢              O                                )                    =                        60          1400                =                  42.9          ⁢                                           ⁢          ms                    
It typically takes five time constants for the circuit to converge to 1% of its steady state value. For the example in FIG. 18B, it will take 214 ms for VP to reach 14.27 volts after going off-hook. Most countries require that the current ICO be above a certain level in a short period of time after going off-hook. For example, in CTR21 countries the current must have settled within 20 ms after going off-hook. During this startup time period, AC transmit and receive modem signals have not yet started. This allows the gyrator cutoff frequency to be extended to higher frequencies at startup. One way to do this is shown in FIG. 18B where a smaller inductor LF is switched in at startup by closing S1 and opening S2. Once steady state has been achieved, the larger inductor LS is switched in by closing S2 and opening S1. The example cutoff frequency Fc and five r at startup are as follows:             F      ⁢                           ⁢      c        =                                        R            ⁢                                                   ⁢            P                    +                      R            ⁢                                                   ⁢            C            ⁢                                                   ⁢            O                                    2          ×          π          ×          L          ⁢                                           ⁢          F                    =              74.3        ⁢                                   ⁢        Hz                        5      ×      τ        =                  5        ×                              L            ⁢                                                   ⁢            F                                (                                          R                ⁢                                                                   ⁢                P                            +                              R                ⁢                                                                   ⁢                C                ⁢                                                                   ⁢                O                                      )                              =                                    5            ×            3                    1400                =                  10.7          ⁢                                           ⁢          ms                    
Thus, conventional DC current levels are set using external components on a country by country basis, resulting in an expensive and inflexible solution.
DC current levels can be set in a digital device such as a gyrator. In a gyrator, a filter operates at the sample rate of the digital device (e.g., at the modem rate). However, the digital transport delay between a digital signal processor (DSP) and its host processor will vary, e.g., because of bus delay, etc. The other device may not give up the bus for 25 mS or more. These transport delays may introduce enough phase delay such that it may not be possible to find one lowpass filter cutoff frequency to make the gyrator stable on all possible central office line terminations. If the gyrator is not stable, the modem will not be able to establish a connection.
There is a need for an improved circuit and method to emulate an inductor in a telephone line interface with a gyrator which will operate with stability quickly and under various country installation conditions and requirements.